Introduction and module overview
Introduction and module overview
Sleep is often taken for granted until it appears to be in short supply or is problematic. The knock-on effect can be significant. Someone who is sleep deprived is not just tired and irritable, but may be suffering from a deterioration in both their physical and mental health.
When it comes to sleeping and being awake, two body systems are at play: sleep/wake homeostasis (sometimes referred to as 'sleep drive') and circadian rhythm (body clock).
The former is largely linked to energy levels; for example, feeling more alert just after waking up. However, what stops a gradual decline in energy levels from waking onwards are environmental cues, such as light, of which sunlight is the most significant.
When light signals are detected by the suprachiasmatic nucleus (SCN) in the brain’s hypothalamus, body temperature starts to rise and cortisol is released, increasing wakefulness.
As the SCN picks up dark signals, body temperature lowers and levels of the hormone melatonin rise, promoting sleep.
Brain activity alters significantly during sleep and passes through various stages.
The four stages:
Rapid eye movement (REM) makes up around one-fifth of sleep. The brain is active – this is when dreaming takes place – but muscles are relaxed. REM is vital for mental and emotional wellbeing
Non-REM stage 1 (NREM1) – also known as pre-sleep – is very light slumber from which it is easy to wake. Body twitches or jerks may be experienced as the heart rate slows, body temperature falls and muscles relax. It comprises around 3-6 per cent of sleep.
NREM2, known as light sleep, due to it being possible to wake without feeling too disorientated, accounts for up to half of sleeping hours.
NREM3, or slow wave sleep, occurs when blood pressure falls and is the most restorative stage. It can be difficult to wake from this phase, which is also when sleep walking or talking is most likely to happen. It makes up around 20-25 per cent of sleep.
All four stages are passed through in cycles, each lasting around 90 minutes (less in infants), with most people having four to five cycles per night. In between, there may be brief moments of waking but these are often not remembered.